Field of the Invention
The present invention relates to a rotary tool for friction stir spot-welding and a friction stir spot-welding method using the rotary tool, and more particularly to a rotary tool which makes it possible to advantageously practice a friction stir spot-welding method to join together a stack of two aluminum members at least one of which is made of an aluminum alloy material containing a large amount of Mg, with a high degree of joint strength, and a friction stir spot-welding method practiced by using the rotary tool to advantageously join together the stack of such aluminum members.
Description of Related Art
There have been recently studied techniques for spot-joining a stack of a plurality of metallic members by a friction stir spot-welding method which utilizes a friction heat to join together aluminum or other metallic members and which is proposed as a joining method that generates a small amount of heat during joining and suffers from low degrees of softening and deformation of the metallic members. It is revealed that a joint formed by the friction stir spot-welding method has a higher degree of joint quality and a sound state of joining with a higher degree of stability as compared with joints formed by conventional joining methods such as a resistive spot-welding method and a rivet joining method.
The friction stir spot-welding method is basically practiced by using a pin-type tool (rotary tool) having a structure in which a hard probe in the form of a pin is provided at a distal end of a tool body in the form of a rod. In the friction stir spot-welding method, the rotary tool is inserted into a stack of desired metallic members, while the rotary tool is rotated at a high speed. The stack is pressed with a shoulder portion consisting of a distal end portion of the tool body, to generate the friction heat between the stack and the shoulder portion and the probe, whereby a plastic flow of materials of the metallic members is caused and a stirred zone is formed around the probe. Thus, the metallic members of the stack are spot-joined at a position where the probe is inserted into the stack. In order to assure a sufficiently high degree of joint strength (welded joint strength) of a weld formed by the friction stir spot-welding method, it is necessary to sufficiently stir a joint interface between the metallic members of the stack.
On the other hand, many proposals have been made regarding the shape of the rotary tool in order to improve a stirring force generated by the rotary tool during a friction stir welding operation. For example, JP-A-2003-326372 proposes a tool with a structure in which the probe takes the form of a screw, and JP-A-2003-326371 proposes a tool with a structure in which various patterns are formed on an end face of the shoulder portion and the probe, while JP-A-2004-58084 proposes a tool with a structure in which a groove having a triangular cross sectional shape is formed over an entire circumference of an end face of the shoulder portion. Further, JP-A-2002-336977 proposes a tool with a structure in which an end face of the shoulder portion is recessed in a direction from its outer periphery toward a proximal portion of the probe, and JP-A-2004-106037 proposes a tool with a structure in which a recess is arcuately formed in an end face of the shoulder portion.
By the way, in a case where a joining operation is performed on a member made of an aluminum alloy material containing Mg in a large amount of not less than 3% (on the mass basis: in the following description, “%” means “% by mass”, unless otherwise specified), such as aluminum alloy materials according to JIS A5154, JIS A5056 and JIS A5182, an oxide film generated during the joining operation contains a large amount of a stiff Mg oxide, so that the oxide film itself becomes stiff. Such an oxide film cannot be easily destructed, so that the presence of the oxide film at the joint interface would result in formation of a whisker-like or hook-like unwelded portion, giving rise to a problem that a sound weld cannot be formed, and the weld is likely to have an insufficient degree of joint strength. Particularly, in the case where the above-described friction stir spot-welding method is practiced, it is necessary to destruct the stiff oxide film with a force for stirring metals of the members to be joined together and to stir the metals of the members directly with each other, in order to assure a sufficiently high degree of joint strength. However, where at least one of the two aluminum members to be joined together is made of an aluminum alloy material containing a large amount of Mg, it was difficult to achieve a sufficiently high degree of joint strength even by using the rotary tools configured to improve the stirring force as described above.